The invention relates to a method of acquiring a three-dimensional image data set of a periodically moving organ of the body of a patient by means of an X-ray device in which projection data sets are acquired by the X-ray device simultaneously with a motion signal related to the periodic motion of the organ. The present invention also relates to an X-ray device including an X-ray source and an X-ray detector for acquiring projection data sets from different X-ray positions and enabling the formation of a three-dimensional image data set of a moving organ of the body of a patient from the projection data sets, and a mechanism for measuring a motion signal related to the periodic motion of the organ which is acquired simultaneously with the acquisition of projection data sets.